Support buildpacks creating an empty release file.
Support buildpacks creating an empty release file.

Ruby 1.9's YAML cannot load empty files, thus we have to test earlier.

Signed-off-by: Michael Elsdoerfer <17b9e1c64588c7fa6419b4d29dc1f4426279ba01@elsdoerfer.com>

progrium/slugbuilder,flynn-archive/slugbuilder,scrapinghub/slugbuilder